Astronauts' sleep in zero-g, i.e. in absence of gravity, is frequently interrupted by the disturbing feeling of being in a state of "free-fall". During non-sleep periods, this feeling can be overcome by a "mental override" which compensates for the unusual physical environment of zero-g.
According to the analysis of Dr. W. Ockels, an ESA Scientist Astronaut, many astronauts have in the past slept continuously for only two to four hours during the first nights in space. Only after re-adjusting their perception of their "actual" state were they able to go back to sleep. Of course, this has a negative effect on their well-being and work effectiveness in space.